When the odds against an event are given as "a:b", the probability of the event can be found by using the formula:
Probability of the event = b / (a + b)
In this case, the odds against the event are 3:7, which means that the probability of the event not occurring is 3/(3+7) or 3/10. Therefore, the probability of the event occurring is:
1 - 3/10 = 7/10
So the probability of the event is 7/10 or 0.7 (70%) when the odds against the event are 3:7.